Dissolved organic matter degradation by sunlight coagulates organo-mineral colloids and produces low-molecular weight fraction of metals in boreal humic waters

Olga V. Oleinikova (1) , Olga Yu. Drozdova , Sergey A. Lapitskiy , Vladimir V. Demin , Andrey Yu. Bychkov , Oleg S. Pokrovsky (1)

1 GET - Géosciences Environnement Toulouse

Photochemical degradation of dissolved organic matter (DOM) is recognized as the major driver of CO₂ emission to the atmosphere from the inland waters of high latitudes. In contrast to numerous studies of photo-induced DOM transformation, the behavior of trace element (TE) during photodegradation of boreal DOM remains virtually unknown. Towards a better understanding of concentration, size fractionation and speciation change of DOM and TE in boreal waters subjected to solar radiation, we conducted on-site photo-degradation experiments in stream and bog water collected from a pristine zone of the Northern Karelia (Russian subarctic). The removal of Fe and Al occurred only in the bog water (90% and 50% respectively, over 5 days of reaction), whereas no detectable decrease of dissolved (<0.22 μm) Al and Fe concentration was observed in the boreal stream. A number of low-soluble TE linked to Fe-rich organo-mineral colloids followed the behavior of Fe during bog water exposure to sunlight: Al, P, Ti, V, Cr, As, Y, Zr, REEs, Hf, Th, Pb and U. The second group of elements (Li, B, Mg, Ca, Sr, Ba, Na, K, Rb, Si, Mn, Ni, Cu, Co, Cd, Sb) was indifferent to photodegradation of DOM and exhibited a non-systematic variation (±10-15% from the control) of <0.22 μm fraction in the course of sunlight exposure. The bog water insolation yielded a factor of 3 ± 1 increase of low molecular weight (LMW < 1 kDa) fraction of organic carbon, Al, Fe, U, Mg, Ca, Mn, Co, Ni, Sr, Cd and Ba after 200 h of sunlight exposure compared to the dark control. The LMW_{< 1 kDa} fraction was preferentially enriched in Fe, Al, Ca, Mg and other divalent metals relative to C_org. The climate warming leading to water temperature rise in the boreal zone will intensify the Fe and Al hydroxide coagulation while increasing the production of LMW organic ligands and free metals and metal - organic complexes.

Mots clés: Photodegradation; Arctic; Iron; Organic carbon; Trace element; Complexation; Size fractionation

Domaines: Planète et Univers [physics]

Olga V. Oleinikova, Olga Yu. Drozdova, Sergey A. Lapitskiy, Vladimir V. Demin, Andrey Yu. Bychkov, et al.. Dissolved organic matter degradation by sunlight coagulates organo-mineral colloids and produces low-molecular weight fraction of metals in boreal humic waters. Geochimica et Cosmochimica Acta, 2017, 211, pp.97-114. ⟨10.1016/j.gca.2017.05.023⟩. ⟨insu-03661368⟩